How do I add a remaining quantity message on my products in Simple theme? And photo question...

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I'll start by saying I'm completely clueless with coding, but have a son who could maybe do anything code related.  
I'm in the Simple theme - I have products with multiple variants, but customers can't see how many of each variant I have and I can't see how to add it in.  I've been shown a box to tick, but on the Supply theme, which isn't there on Simple.  

And my photo question - is getting them all the same size on the main page down to my photo sizing, or is there something that can be added to the theme to adjust them for me?

Any help much appreciated, thank you!